um1 bad accuracy

[l_tonic 0]

Hello UM Community

i got a problem that my UM1 always prints 0.2mm too little on x/y axis but on the y axis it prints 0.4mm too little so my prints, which are mostly used for mechanical systems, get unaccurate and dont fit.

so instead of 10mm on x it prints 9.7 - 9.8 mm and instead of 15mm on y 14.5 -14.6 mm

i looked for the y/x Steps per mm settings in my UM controller which are set both on 078.74 but i have no idea what that means or how i could change it with positive effects

how can i kill most of the diffrence between the axis and also make it more accurate?

lg l_tronic

 

[gr5 2,026]

78.74 steps/mm means if you ask the X or Y axis to move 1mm the controller sends 78.74 steps to the servo. This seems very straightforward to me. I don't know how to make it clearer. If you want to increase the scale (print bigger) on an axis by 3% then you want to increase the steps by 3%.

But I don't recommend this. The steps/mm is worked out pretty carefully based on diameters of pulleys and if you mess with it, the software endstops might not work if you print something at the full 205mm size.

The shrinkage you see is caused by several effects. I'd have to see the part to know exactly what the problem is. But PLA shrinking by 3% sounds about right. .25/10mm (your x axis) is 2.5% shrinkage. .45/10 is 3% shrinkage. It sounds like you have identical shrinkage on both axes.

So alternatively you could scale up all parts by 2.5 to 3% every time you load a part in Cura.

But in practice I think you will find that there are many other factors that change the dimensions of parts. For example vertical holes through a part are always smaller due to 3 factors: PLA shrinkage, the pulling effect as it lays down a "string" of PLA around the outside circle pulls it inward and the fact that circles are done in CAD with a polygons and these are inscribed *inside* the circle. So less than 10 sided polygons noticably shrink the hole diameter.

The best solution is to always print a part twice - or print small test sections, then increase the sizes in your CAD to compensate. Unfortunately different brands and colors of plastic will need different compensations and the way you print (temperature, fan settings, speed) will also change these so it's not something that you can share with other people.

 

[l_tonic 0]

sounds good thank you for your explanation!

but even if i print a cube with 1cm x 1cm x 1cm the y axis is still 0.2-0.3mm smaller than the x axis and that happens every time (note: the difference de or increase with the size of the object) so f.e. my screw holes get egg shaped

 

[Dim3nsioneer 557]

The steps per mm value can be slightly different according to my own experience. I derived the 'correct' number for my UM1 by moving 150mm or 200mm and measuring the real distance. Maybe this works as well for you?

 

[gr5 2,026]

Ah - well there is a 4th source for error! You probably have "play" or "backlash". More information here (posts 7,8):

http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/?p=14474

You probably need to tighten your Y axis short belt - loosen the motor mounts, slide it down tight, retighten motor mounts.

Although play can also be caused by very high friction which you would notice being different for X versus Y when pushing the head around with power off. Friction most often caused by end caps on the 4 rods.

 

[IRobertI 516]

I guess I'll throw in another possible (but unlikely in this case I think) variable while we're at it. Uneven cooling causing uneven shrinkage.

 

[l_tonic 0]

nope i have double fans intalled so cooling cannot be the reason

but thanks to you for replying i will try it out

 

[gr5 2,026]

Uneven does not necessarily mean left/right uneven. It could be up/down uneven due to upper layers being warmer other issues. I don't have anything specific in mind. I don't know if Robert had anything specific.

 

[IRobertI 516]

No, nothing specific, I just figured I'd add another one to the growing list for giggles. In hind sight I probably should've kept my mouth shut.

 

[jameshs 42]

No, nothing specific, I just figured I'd add another one to the growing list for giggles. In hind sight I probably should've kept my mouth shut.

except it is a valid point - you merely said cooling - and I bet the 'double fan' has both on the X axis - i.e. the x axis is double cooled and the Y axis not at all except from the x axis.

I only noticed this when I was asked t print a 'marvin' as a sample for 3Dhubs - and I found that the right hand eyebrow was different to the left. As this was a print sample to prove print competence I was keen to improve it and watched the printer like a hawk - the eyebrow nearer the fan did not suffer from (a) initial cooling lag and (b) reheat from the returning nozzle (next layer)

So I turned the model on the buildplate so both eyebrows received the same fan axis and voila - they ended up the same.

So IMHO there is a x/y difference from cooling in the fan axis, and having two would probably maintain/magnify this -artefact - possibly in the initial deposition cooling and then in the reheating from the next layer.

To experiment - drop the fan speed and see if it has a difference, or fans off - anything to make the x/y less different.

so IMHO a good observation! - follow the differences.

James